The technical field of the invention is that of microactuators intended to perform mechanical, chemical, electrical, thermal or fluidic functions in microsystems, for microelectronics applications such as chips or biomedical applications such as analysis cards incorporating microfluids technology or chemical synthesis applications such as microreactors.
Microactuators are miniaturized objects machined in solid supports that may be semiconductors or insulators, with a view to forming microsystems such as, for example, microvalves or micropumps in fluid microcircuits, or microswitches in electronic microcircuits.
Microactuators using electrostatic, piezoelectric, electromagnetic and biometallic effects have been in existence for some time now. A new generation of microactuator is just beginning to appear: those employing a pyrotechnic effect. On this subject, patent WO 98/24719 describes a miniature valve for filling the reservoir of a transdermal administration appliance. The principle of operation of this valve relies on the fragmentation of a substrate caused by the gases of combustion of a pyrotechnic charge, said substrate initially separating a reserve of fluid from an empty reservoir. This microvalve may, according to another alternative form of embodiment of the invention, be used with an inflatable envelope. The combustion gases first of all rupture the substrate then inflate the envelope with a view to driving a fluid out. These microvalves have the twofold disadvantage of emitting fragments of substrate into the microcircuit and of mixing the combustion gases with the fluid they are supposed to release.
In general, the microactuators used in microcircuits need to offer good performance in terms of the forces they deliver, to maintain a small size, and to remain an entire and autonomous entity while they are operating, without the possibility of breaking into pieces in order to avoid emitting particles into the microcircuit into which they are built, and without the possibility of the combustion gases contaminating said microcircuit. In the case of a fluid microcircuit, the addition of pyrotechnics allows the microactuators to generate pressure forces 100 to 1000 times as high as those produced by microactuators operating on a piezoelectric or electrostatic source. In addition, the gases emitted by the combustion of the pyrotechnic charge may also serve to heat a fluid or part of a micromechanism without mixing with it.
The microactuators according to the invention meet these three requirements.
The subject of the present invention is a microactuator comprising a chamber produced in a solid support and containing a pyrotechnic charge, characterized in that the chamber is partially delimited by a deformable membrane so that the gases emitted by the combustion of the pyrotechnic charge allow the volume of said chamber to increase by deforming said membrane, while at the same time leaving the solid walls of the chamber intact.
In other words, the gases emitted by the combustion of the pyrotechnic charge have no influence on the geometry of the solid part of the chamber, either in terms of deforming the walls or in terms of fragmenting them.
These microactuators may by themselves perform functions within a microcircuit, such as, for example, exerting pressure on a fluid to help displace it in order to remove it, or closing a fluid duct by deforming the membrane, but they are more usually intended to be included in microsystems.
A microsystem is a miniaturized multifunction device, the maximum dimensions of which do not exceed a few millimeters. In the case of a fluid microcircuit, a microsystem may, for example, be a microvalve or a micropump and, in the context of an electronic microcircuit, may be a microswitch or a microbreaker. Microactuators are produced in semiconductor supports, such as those made of silicon for example, in the case of a microelectronics application. They may be designed in other materials, such as polycarbonate, for other applications, particularly in the biomedical field. The configuration of the chamber is such that, under the effect of the gases emitted by the combustion of the pyrotechnic charge, it increases in volume. The chamber may contain several pyrotechnic charges, not with a view to increasing the internal pressure in said chamber by firing said charges simultaneously, but so as to maintain a pressure level that is fairly constant over time in order to guard against possible premature relaxation of the chamber, particularly in the case of micropumps. In this case, the charges are initiated sequentially, at predetermined time intervals. As a preference, said chamber defines a hermetic space once it has expanded. In other words, once combustion is over, the chamber remains in a configuration corresponding to a state of maximum expansion.
Advantageously, the pyrotechnic charge consists of a composition based on nitrocellulose. What actually happens is that because of the very small size of the pyrotechnic charges used, their mass not exceeding a few micrograms, it is particularly desirable for homogeneous compositions to be used.
According to another preferred embodiment of the invention, the pyrotechnic charge consists of glycidyl polyazide.
As a preference, the volume of the chamber is less than 1 cm3. Advantageously, the fill density, which is the ratio of the mass of the pyrotechnic charge to the volume of the chamber, is between 0.01 μg/mm3 and 0.1 mg/Mm3. For a given chamber volume it is entirely possible to define a pyrotechnic charge, in terms of mass, geometry and composition, that is able to produce a given amount of energy.
According to a first preferred embodiment of the invention, the pyrotechnic charge is deposited on a heating conducting track and advantageously has a deposition thickness of less than 200 μm.
According to a second preferred embodiment of the invention, the pyrotechnic charge coats a heating conducting wire passing through the chamber, the diameter of said wire being between 10 μm and 100 μm. Although these two modes of initiation in most cases allow the pyrotechnic charge to be fired, it has, however, been found that in certain configurations there is a problem associated with thermal losses by conduction due to the heating conducting element's coming into contact with the support, these losses requiring additional energy in order to be able to fire the charge, this generally being accompanied by significant heating of the microactuator, which is not always desirable. Thus, in a third preferred embodiment of the invention, the heating conducting track is deposited on the pyrotechnic charge using techniques that have been widely proven in the field of microcircuits such as, for example, the deposition of paint or conducting ink by screen printing or inkjet printing, so as to avoid any direct contact between said conducting track and the substrate. According to a fourth preferred embodiment of the invention, the chamber has a cavity hollowed into the support and said pyrotechnic charge is in the form of a film covering said cavity in order, here, again, to reduce or even eliminate thermal losses by conduction by isolating the pyrotechnic charge from any heat-conducting solid support. For this last configuration, use may be made of high-energy materials with a film-forming capability, such as collodion for example.
However, the optimized configuration for best solving the problem associated with thermal losses by conduction is to deposit the pyrotechnic charge in the form of a film over a cavity of the support and to fire it using a heating conducting track that is itself deposited on said charge. By doing this, direct contact between the heating track and the support are non-existent and those between the charge and said support are almost non-existent.
Because of the miniaturizing of the pyrotechnic charge, its firing system has itself to be of small bulk, while at the same time remaining highly reliable. More generally, it is also possible to fire the pyrotechnic charge using other means, particularly those involving either a piezoelectric crystal, or a striker, provided that they meet the twofold requirement of miniaturization and reliability, or using a laser beam, pyrotechnic energy [lacuna] along a waveguide or an optical fiber.
As a preference, the chamber is partially delimited by a flexible membrane able to inflate under the effect of the gases emitted by the pyrotechnic charge. The extendibility properties of the membrane may differ according to the requirements associated with the use of the actuator.
According to another preferred embodiment of the invention, the chamber is partially delimited by a flexible membrane folded in said chamber, said membrane being able to unfold under the effect of the gases emitted by the pyrotechnic charge. Depending on the configuration, the membrane may either be folded onto itself, or folded up in the chamber. Advantageously, once the membrane is unfolded under the effect of the gases, the final volume of the chamber is greater than its initial volume. As a preference, the membrane is made of Teflon. Advantageously, for microelectronics applications, the membrane may be entirely or partially covered with a conducting material.
The invention also relates to a microsystem including a microactuator according to the invention, characterized in that the deformation of the membrane causes a solid part to move. Indeed the gases emitted by the combustion of the pyrotechnic charge create a raised pressure in the chamber, which will tend to expand by deformation of the membrane. The membrane then comes into contact with a part placed near the microactuator, and when the pressure forces reach a threshold value they cause said part to move.
According to a first preferred embodiment of a microsystem according to the invention, the solid part is able to obstruct a fluid duct after the pivoting of said part under the effect of the combustion gases. For this configuration, in which the microactuator is used in the context of a fluid microcircuit, the microsystem may be likened to a closure microvalve.
According to a second preferred embodiment of the invention, the solid part closes off a fluid duct and the movement of said part by pivoting causes said duct to open. For this configuration, the microsystem may be likened to an opening microvalve.
According to a third preferred embodiment of the invention,
i) a flexible membrane is situated in an annular space that may be likened to a groove,
ii) the pyrotechnic charge is situated in an annular space that may be likened to a groove smaller than the one in which the flexible membrane is situated and positioned concentrically with respect to that groove, the two grooves communicating with each other via at least one opening,
iii) a flat solid part bears against the support, covering the annular space in which the flexible membrane is situated, said part itself being covered by an elastic membrane and blocking off a fluid duct,
so that the gases emitted by the combustion of the charge lead to deployment of the flexible membrane situated in the annular space and cause the flat part to move, causing fluid to be drawn up into the space that the elastic membrane creates as it moves away from the support.
For this configuration, the microsystem may be likened to a vacuum micropump and the use of several pyrotechnic charges fired in sequence may seem particularly appropriate, in order to maintain a minimum threshold pressure level for a certain length of time and thus avoid premature natural reflux of the fluid.
According to a fourth preferred embodiment of the invention, the membrane deforms under the effect of the combustion gases to obstruct a fluid duct. Advantageously, the chamber is partially delimited by a membrane that is bistable such that said membrane, initially concave, becomes convex under the effect of the gases emitted by the charge. For this configuration, the microsystem, which acts like a closure microvalve, moves no part and is coincident with the microactuator. Advantageously, the element that obstructs the fluid duct, whether this be the flat, solid part or the bistable membrane, is surmounted by a flexible protrusion to ensure a good seal at the closure of said duct, said protrusion being something that can be likened to a plug.
The microactuator according to the invention may be used in electronic microcircuits by contributing to the production of Microsystems such as microswitches or microbreakers. What happens is that the membrane that partially delimits the chamber and is entirely or partially covered with a conducting material can inflate or deploy in such a way as to close or to open an electric microcircuit. Likewise, the microactuator according to the invention equipped with a non-conducting flexible membrane can move a conducting solid part so as to close or to open an electric microcircuit or perform the dual function of first of all opening an electric microcircuit and then closing another one.
The pyrotechnic microactuators according to the invention have the advantage of having good performance and good reliability while at the same time remaining clean. They are clean in two respects: first, they remain intact throughout their phase of operation without the risk of being fragmented, avoiding the release of parasitic solid particles into the microcircuit, and second, the gases emitted by the pyrotechnic charge are trapped in the chamber, which delimits a gastight space, without any possibility of invading the microcircuit. In addition, the pyrotechnic microactuators according to the invention are simple. A chamber with a membrane, a pyrotechnic charge, and an ignition system are their only constituent elements, and the physico-chemical phenomena they generate remains basic.
Finally, for a given chamber volume, the great variation in pyrotechnic compositions that can be incorporated into the microactuators according to the invention makes it possible to obtain a very wide range of initiating arrangements, suiting it to a great many configurations.